In general, polyurethane paint compositions can be classified into two types. In the first type the two components of the urethane, namely the isocyanate-reactive an the isocyanate-containing component are mixed together in a single container. In these systems, the isocyanate groups are blocked by blocking agents such as alcohols, oximes, amides, phenols, and ketoximes, among other blocking agents in order to prevent reaction between the isocyanates and the isocyanate-reactive component during storage. To apply these coatings the mixture is applied to a substrate and heated above the blocking temperature to unblock the isocyanates and render them reactive. This, in turn, causes curing of the coating by the ensuing reaction between the unblocked isocyanates and the isocyanate-reactive component.
The second type of polyurethane composition is a two component system which is packaged in two separate containers. In this system the isocyanate component is stored in a separate container from the isocyanate-reactive component. Typically, the isocyanate component contains only free isocyanates since it is physically isolated from the isocyanate-reactive component by storage in separate containers. This system has the advantage of curing at low temperatures because there is no need to heat the composition to unblock the isocyanates. However, this type of urethane composition has the disadvantage that, during application, workers are exposed to the free isocyanates which are highly toxic.
The two component, single-container system is generally easier to apply since no mixing is required. Further, since the isocyanate groups are blocked, the toxicity associated with application of the polyurethane coating is minimized because the exposure of workers to free isocyanates is substantially reduced. Examples of these types of urethane systems can be found in U.S. Pat. Nos. 4,439,593; 4,412,034; 4,410,689; 4,410,769; 4,456,740; 4,442,146 and 4,456,738. In these systems, the isocyanate groups are blocked by blocking agents such as alcohols, oximes, amides, phenols and ketoximes, among other blocking agents. These compositions generally cure at high temperatures. Blocked isocyanates blocked with other blocking agents and having low curing temperatures are generally unstable under standard storage conditions when in admixture with isocyanate-reactive compounds such as polyhydroxylates and thus are not generally useful in the single container system.
The blocked isocyanate-containing polyurethane compositions which are packaged in a single container are cured by displacing the blocking agents with a co-reactant at temperatures of about 120.degree. C. to 250.degree. C. Upon curing, the blocking agent is released and usually volatilizes from the coating. The resulting free isocyanates are then able to react with active isocyanate-reactive compounds to form thermally stable urethane or urea linkages. In general, blocked aromatic polyisocyanates have lower dissociation temperatures than blocked aliphatic polyisocyanates. Further, the dissociation temperatures of blocked isocyanates is directly affected by the particular blocking agent employed. The most significant problem with two-component, single container polyurethane coating is that they generally cure at relatively high temperatures in excess of 120.degree. C. All two-component single container coatings curing at temperatures below 120.degree. C. generally exhibit poor storage stability and the lower the curing temperature the less storage stability the urethane will have.
An example of a two-component, two-container urethane system is described in U.S. Pat. No. 3,385,829. These two-container systems typically contain free isocyanates, cure at low temperatures and produce high gloss, durable films which possess good solvent and chemical resistance. These properties, along with their hardness, have made two-container polyurethane coatings a standard for aircraft and automotive finish and maintenance applications.
In use, these two-component, two-container polyurethanes are mixed just prior to their application. Typically, these two container systems have a limited pot life after mixing during which the mixture retains its liquid form. Thereafter, curing occurs at room temperature as urethane linkages begin formation by reaction of the free isocyanates with the isocyanate-reactive componet.
Although these two-component, two container urethane systems are advantageous because of their low curing temperatures, they exhibit significant drawbacks as well. For example, the presence of free isocyanates in the mixtures is highly undesirable since such free isocyanates are extremely toxic, particularily with respect to the personnel charged with their application. In addition, these two-component, two-container systems often continue to harden after curing and result in brittle, inflexible cured coatings. Such coatings exhibit poor resistance to chipping and are often difficult to repair. In addition, great care must be employed in the handling of the isocyanate portion since it will react with water including moisture from the surrounding air. A nitrogen blanket is thus frequently required to isolate the isocyanate-containing portion from direct contact with air.
Accordingly, there is a need in the art for a polyurethane coating system exhibiting reduced toxicity, low curing temperatures, and which produces cured coatings of high flexibility that are not brittle and are easy to repair.